The Nurses's Health Study is a prospective cohort study of 121,700 female nurses that began in 1976. As part of our 1986 application for a five-year extension of the study (CA 40356), we included a specific aim "to evaluate the feasibility of collecting blood and urine specimens from the total cohort to assess biologic markers of exposure or predisposition to a variety of cancers" and "to evaluate the effects of transport and storage on the reproducibility of specific biochemical determinations in blood and urine." In our feasibility study, we evaluated several options for specimen collection, and determined that the plasma sex hormones, lipid fractions, fatty acids, vitamins of primary interest as well as white cell DNA would be stable in specimens sent with an ice pack by overnight mail. We propose to collect blood samples from 48,000 women participating in the Nurses' Health Study who are free from cancer and heart disease and who have previously provided detailed dietary assessment, and a wealth of other information on height, weight, smoking, reproductive and medical history, exogenous hormone use, and other variables. The ongoing Nurses' Health Study will provide followup and documentation of cancers and cardiovascular outcomes. We have limited our request to blood rather than urine specimens since the variables of primary interest could be measured best (or exclusively) with blood. These specimens will be used in nested case-control studies in years 4 and 5 of this proposal to test a series of specific hypotheses in these general areas: 1. Plasma sex hormone levels (total and free estradiol, estrone,prolactin, testosterone, and androstenedione and progesterone) in relation to risk of breast cancer among postmenopausal women. An additional component of the study will be to quantify laboratory and biologic variability of sex hormone levels among a sample of 60 women. 2. Plasma levels of beta-carotene, vitamin E, and omega-3 fatty acids and risk of breast and colon cancer; exposure to elevated blood galactose levels and risk of ovarian cancer. 3. Specific lipoprotein fractions, apo-proteins, and fatty acids and risk of coronary heart disease (CHD) and stroke in women. 4. High levels of drinking water calcium and magnesium and risk of CHD and stroke.